1. Field of the Invention
This invention relates to a new method of making superconducting lead-alloy Josephson junctions. More particularly, this invention relates to a method of making lead-alloy based electrodes for Josephson junction devices which exhibit a high degree of thermal cyclability.
2. Description of the Prior Art
Numerous papers have been published concerning problems that arise during fabrication and use of lead-alloy for Josephson tunnel junction devices. Lead-alloy-based Josephson devices are described as being weak and subject to degradation when exposed to thermal cycling between their supercool operating environment temperature and room temperature. Lead-alloy-based Josephson devices are also described as being prone to failure when annealed at temperatures in excess of around seventy degrees centigrade even though proper processing of Josephson junction devices exposes the wafers to steps which are best performed at temperatures in excess of seventy-five degrees centigrade.
The underlying problem arises from the deposited lead and lead-alloy material used in making the electrodes of the Josephson junction device. The layers are first deposited as discrete metal layers in laminate form. Then the layers are interdiffused and annealed at elevated temperatures to provide a homogeneous alloy having distinctive polycrystalline grain-like structure. Thermal cycling of this grain-like structure results in growth of the grains in the electrode material. In Josephson junction devices, the very thin tunnel barrier is formed intermediate the base electrode and the counter electrode. The enlargement of the grains of the base electrode or counter electrode are known to cause penetration of the very thin tunnel barrier which results in failure of the Josephson junction devices.
H. C. Ward Huang, et al described in the IEEE Transactions on Electron Devices at volume Ed-27, number 10 October, 1980 at pages 1979 to 1987 a process for making lead-indium-gold alloy base electrodes that exhibit improved thermal cycling stability. In the preferred method described in this article, the base electrode materials are deposited at seventy-seven degrees Kelvin to produce a fine-grain structure.
Josephson junction devices are operated at or near four degrees Kelvin. Circuits embodying Josephson junction devices are repaired and maintained at room temperatures around three hundred degrees Kelvin. In the preferred method of making Josephson junction devices, process temperatures in the range of three hundred and seventy degrees Kelvin can be encountered. Presently, thermal cycling of Josephson junction lead-alloy based electrodes between operating temperatures and room temperatures and/or process temperatures has lead to high rejection rates of Josephson junction devices as a result of the production process and further have resulted in premature failure of acceptable devices due to thermal cycling.
Presently, Josephson junction devices are produced at relatively low temperatures because it is known that evaporation of lead at pressures less than 10.sup.-7 torr produce a desirable fine-grain deposited structure. At pressures less than 10.sup.-7 torr, most of the oxygen, oxides of carbon and water vapor are removed so that the grains are free of oxides which would enlarge the grain size.
Metals such as zinc, aluminum and manganese, etc. have been vapor deposited as particles in inert gas environments in order to study homogeneous nucleation and the effect of atmospheric polution. These studies were concerned with particle size and the growth of particles but were not concerned with controlling grain size or producing superconducting layers for Josephson junction devices.
It is known that abnormal size grains or large grains of superconducting base electrode material of Josephson junction devices tend to enlarge when thermally cycled and inevitably result in penetrating the tunnel barrier formed on top of the base electrode material.
It would be desirable to produce a lead-alloy based electrode material for Josephson junction devices which is substantially immune to thermal cycling and which can be made at room temperatures and which can be processed at temperatures well in excess of seventy-five degrees centigrade (348 degrees Kelvin).